A hamster monoclonal antibody has been developed that is specific for a 161 kDa protein expressed on mouse mast cells and some activated macrophages. The protein is not found on most other mature hematopoeitic cells and thus can be used to distinguish mast cells from basophils. Among FcepsilonR+ cells, it is generally co-expressed with c-kit. p161 has been purified to homogeneity and sequences of several proteolytic peptides have been obtained. These sequences reveal a high degree of homology with human and rat CD13. Using a rat cDNA clone for CD13, a full-length mouse CD13 clone has been obtained. Stable transfectants expressing mouse CD13 are K-1 positive verifying that CD-13 is p161. CD13 is a neutral aminopeptidase, raising the possibility that it has an important role in mast cell function. The availability of this antibody has made possible the identification of a set of mast cells that lack FcepsilonRI. These cells can be grown in long-term culture in IL-3; they contain histamine, express FcgammaRII and have Alcian blue positive granules. They express mRNA for the FcepsilonRI alpha and beta chains but lack gamma chain mRNA. Transfection of gamma chain into FcepsilonRIneg mast cells allows the expression of FcepsilonRI. The expressed receptor is functional in that cross-linking it leads to secretion of IL-3. Expression of CD13 on FcepsilonRIpos cells allows a discrimination of mast cells and basophils. Short-term-cultured basophils produce IL-4 in response to cross-linkage of FcepsilonRI, but little IL-13 mRNA. By contrast, cultured mast cells are excellent IL-13 producers, but secrete little IL-4. Thus, the two major FcepsilonRIpos cells preferentially secrete distinct cytokines, whose actions, while overlapping, tend to affect different cell types. Thus, these cells may prove to have unique roles in mediating allergic inflammatory responses.